Pregnancy offers a unique experimental situation for studying blocking or control factors that can exert an inhibitory influenc on the development and deployment of cellular immunity to transplantation antigens and possible facilitates the induction of tolerance in adult subjects. The placenta affords a unique "biological filter" allowing transmission of suppressed but potentially reactive immunocytes, but not of the cells that make blocking antibody. Consequently, althou blocking antibodies also cross the placenta, their cellular source in not transmitted. By employing (1) lymphoid cell transfers to pregnant animals (2) zygote transfer to tolerant animals and (3) parabiotic experiments using pregnant subjects experiments will be performed to define the role of humoral factors in the abrogation of graft-versus-host disease and in immunological tolerance.